1. Lecture 4: Classes (continued) June 14, 2004
IS Program Design and Software Tools Introduction to C++ Programming
Lecture 4: Classes
(continued)
June 14, 2004

2. Using Set and Get Functions
Set functions
Perform validity checks before modifying private data
Notify if invalid values
Indicate with return values
void Time::setHour( int h ) {
hour = ( h >= 0 && h < 24 ) ? h : 0;
} // end function setHour
Get functions
“Query” functions
Control format of data returned
int Time::getHour()
return hour;

3. Subtle Trap: Returning a Reference to a private Data Member
Reference to object
&pRef = p;
Alias for name of object
Lvalue
Can receive value in assignment statement
Changes original object
Returning references
public member functions can return non-const references to private data members
Client able to modify private data members

4. // Fig. 6.21: time4.h
// Declaration of class Time.
// Member functions defined in time4.cpp 4
// prevent multiple inclusions of header file
#ifndef TIME4_H
#define TIME4_H 8
class Time { 10
11 public:
Time( int = 0, int = 0, int = 0 );
void setTime( int, int, int );
int getHour(); 15
int &badSetHour( int ); // DANGEROUS reference return 17
18 private:
int hour;
int minute;
int second; 22
23 }; // end class Time 24
25 #endif
time4.h (1 of 1)
Function to demonstrate effects of returning reference to private data member.

5. Return reference to private data member hour.
25 // return hour value
26 int Time::getHour()
27 {
return hour; 29
30 } // end function getHour 31
32 // POOR PROGRAMMING PRACTICE:
33 // Returning a reference to a private data member.
34 int &Time::badSetHour( int hh )
35 {
hour = ( hh >= 0 && hh < 24 ) ? hh : 0; 37
return hour; // DANGEROUS reference return 39
40 } // end function badSetHour
time4.cpp (2 of 2)
Return reference to private data member hour.

6. badSetHour returns reference to private data member hour.
// Fig. 6.23: fig06_23.cpp
// Demonstrating a public member function that
// returns a reference to a private data member.
#include <iostream> 5
using std::cout;
using std::endl; 8
// include definition of class Time from time4.h
10 #include "time4.h" 11
12 int main()
13 {
Time t; 15
// store in hourRef the reference returned by badSetHour
int &hourRef = t.badSetHour( 20 ); 18
cout << "Hour before modification: " << hourRef; 20
// use hourRef to set invalid value in Time object t
hourRef = 30; 23
cout << "\nHour after modification: " << t.getHour(); 25
fig06_23.cpp (1 of 2)
badSetHour returns reference to private data member hour.
Reference allows setting of private data member hour.

7. fig06_23.cpp (2 of 2) fig06_23.cpp output (1 of 1)
// Dangerous: Function call that returns
// a reference can be used as an lvalue!
t.badSetHour( 12 ) = 74; 29
cout << "\n\n*********************************\n"
<< "POOR PROGRAMMING PRACTICE!!!!!!!!\n"
<< "badSetHour as an lvalue, Hour: "
<< t.getHour()
<< "\n*********************************" << endl; 35
return 0; 37
38 } // end main
Can use function call as lvalue to set invalid value.
fig06_23.cpp (2 of 2) fig06_23.cpp output (1 of 1)
Hour before modification: 20
Hour after modification: 30
*********************************
POOR PROGRAMMING PRACTICE!!!!!!!!
badSetHour as an lvalue, Hour: 74
Returning reference allowed invalid setting of private data member hour.

8. Default Memberwise Assignment
Assigning objects
Assignment operator (=)
Can assign one object to another of same type
Default: member-wise assignment
Each right member assigned individually to left member
class Date {
public:
Date( int = 1, int = 1, int = 1990 ); // default constructor
void print();
...
}; // end class Date
Passing, returning objects
Objects passed as function arguments
Objects returned from functions
Copy constructor

9. const (Constant) Objects and const Member Functions
Principle of least privilege
Only allow modification of necessary objects
Keyword const
Specify object not modifiable
Compiler error if attempt to modify const object
Example
const Time noon( 12, 0, 0 );
Declares const object noon of class Time
Initializes to 12

10. const (Constant) Objects and const Member Functions
Member functions for const objects must also be const
Cannot modify object
Specify const in both prototype and definition
Prototype: After parameter list
void printUniversal() const;
Definition: Before beginning left brace
void printUniversal() const { .. }
Constructors and destructors
Cannot be const
Must be able to modify objects

11. const (Constant) Objects and const Member Functions
Member initializer syntax
Initializing with member initializer syntax
Can be used for
All data members
Must be used for
const data members
Data members that are references
Constructors and destructors
Cannot be const
Must be able to modify objects
Constructor
Initializes objects
Destructor
Performs termination housekeeping

12. fig07_04.cpp (1 of 3) 1 // Fig. 7.4: fig07_04.cpp
// Using a member initializer to initialize a
// constant of a built-in data type.
#include <iostream> 5
using std::cout;
using std::endl; 8
class Increment { 10
11 public:
Increment( int c = 0, int i = 1 ); // default constructor 13
void addIncrement() {
count += increment; 17
} // end function addIncrement 19
void print() const; // prints count and increment 21
fig07_04.cpp (1 of 3)

13. Declare increment as const data member. fig07_04.cpp (2 of 3)
22 private:
int count;
const int increment; // const data member 25
26 }; // end class Increment 27
28 // constructor
29 Increment::Increment( int c, int i )
: count( c ), // initializer for non-const member
increment( i ) // required initializer for const member
32 {
// empty body 34
35 } // end Increment constructor 36
37 // print count and increment values
38 void Increment::print() const
39 {
cout << "count = " << count
<< ", increment = " << increment << endl; 42
43 } // end function print 44
Declare increment as const data member.
fig07_04.cpp (2 of 3)
Member initializer list separated from parameter list by colon.
Member initializer syntax can be used for non-const data member count.
Member initializer syntax must be used for const data member increment.
Member initializer consists of data member name (increment) followed by parentheses containing initial value (c).

14. Composition: Objects as Members of Classes
Class has objects of other classes as members
Construction of objects
Member objects constructed in order declared
Not in order of constructor’s member initializer list
Constructed before enclosing class objects (host objects)

15. date1.h (1 of 1) 1 // Fig. 7.6: date1.h 2 // Date class definition.
// Member functions defined in date1.cpp
#ifndef DATE1_H
#define DATE1_H 6
class Date { 8
public:
Date( int = 1, int = 1, int = 1900 ); // default constructor
void print() const; // print date in month/day/year format
~Date(); // provided to confirm destruction order 13
14 private:
int month; // 1-12 (January-December)
int day; // 1-31 based on month
int year; // any year 18
// utility function to test proper day for month and year
int checkDay( int ) const; 21
22 }; // end class Date 23
24 #endif
date1.h (1 of 1)

16. date1.cpp (1 of 3) 1 // Fig. 7.7: date1.cpp
// Member-function definitions for class Date.
#include <iostream> 4
using std::cout;
using std::endl; 7
// include Date class definition from date1.h
#include "date1.h" 10
11 // constructor confirms proper value for month; calls
12 // utility function checkDay to confirm proper value for day
13 Date::Date( int mn, int dy, int yr )
14 {
if ( mn > 0 && mn <= 12 ) // validate the month
month = mn; 17
else { // invalid month set to 1
month = 1;
cout << "Month " << mn << " invalid. Set to month 1.\n"; } 22
year = yr; // should validate yr
day = checkDay( dy ); // validate the day 25
date1.cpp (1 of 3)

17. // Fig. 7.8: employee1.h
// Employee class definition.
// Member functions defined in employee1.cpp.
#ifndef EMPLOYEE1_H
#define EMPLOYEE1_H 6
// include Date class definition from date1.h
#include "date1.h" 9
10 class Employee { 11
12 public:
Employee(
const char *, const char *, const Date &, const Date & ); 15
void print() const;
~Employee(); // provided to confirm destruction order 18
19 private:
char firstName[ 25 ];
char lastName[ 25 ];
const Date birthDate; // composition: member object
const Date hireDate; // composition: member object 24
25 }; // end class Employee
employee1.h (1 of 2)
Using composition; Employee object contains Date objects as data members.

18. employee1.h (2 of 2) employee1.cpp (1 of 3)26
27 #endif
employee1.h (2 of 2) employee1.cpp (1 of 3)
// Fig. 7.9: employee1.cpp
// Member-function definitions for class Employee.
#include <iostream> 4
using std::cout;
using std::endl; 7
#include <cstring> // strcpy and strlen prototypes 9
10 #include "employee1.h" // Employee class definition
11 #include "date1.h" // Date class definition 12

19. Output to show timing of constructors.
13 // constructor uses member initializer list to pass initializer
14 // values to constructors of member objects birthDate and
15 // hireDate [Note: This invokes the so-called "default copy
16 // constructor" which the C++ compiler provides implicitly.]
17 Employee::Employee( const char *first, const char *last,
const Date &dateOfBirth, const Date &dateOfHire )
: birthDate( dateOfBirth ), // initialize birthDate
hireDate( dateOfHire ) // initialize hireDate
21 {
// copy first into firstName and be sure that it fits
int length = strlen( first );
length = ( length < 25 ? length : 24 );
strncpy( firstName, first, length );
firstName[ length ] = '\0'; 27
// copy last into lastName and be sure that it fits
length = strlen( last );
length = ( length < 25 ? length : 24 );
strncpy( lastName, last, length );
lastName[ length ] = '\0'; 33
// output Employee object to show when constructor is called
cout << "Employee object constructor: "
<< firstName << ' ' << lastName << endl; 37
employee1.cpp (2 of 3)
Member initializer syntax to initialize Date data members birthDate and hireDate; compiler uses default copy constructor.
Output to show timing of constructors.

20. Output to show timing of destructors.
38 } // end Employee constructor 39
40 // print Employee object
41 void Employee::print() const
42 {
cout << lastName << ", " << firstName << "\nHired: ";
hireDate.print();
cout << " Birth date: ";
birthDate.print();
cout << endl; 48
49 } // end function print 50
51 // output Employee object to show when its destructor is called
52 Employee::~Employee()
53 {
cout << "Employee object destructor: "
<< lastName << ", " << firstName << endl; 56
57 } // end destructor ~Employee
employee1.cpp (3 of 3)
Output to show timing of destructors.

21. Create Date objects to pass to Employee constructor.
// Fig. 7.10: fig07_10.cpp
// Demonstrating composition--an object with member objects.
#include <iostream> 4
using std::cout;
using std::endl; 7
#include "employee1.h" // Employee class definition 9
10 int main()
11 {
Date birth( 7, 24, 1949 );
Date hire( 3, 12, 1988 );
Employee manager( "Bob", "Jones", birth, hire ); 15
cout << '\n';
manager.print(); 18
cout << "\nTest Date constructor with invalid values:\n";
Date lastDayOff( 14, 35, 1994 ); // invalid month and day
cout << endl; 22
return 0; 24
25 } // end main
fig07_10.cpp (1 of 1)
Create Date objects to pass to Employee constructor.

22. Destructor for Employee’s member object hireDate.
Date object constructor for date 7/24/1949
Date object constructor for date 3/12/1988
Employee object constructor: Bob Jones
Jones, Bob
Hired: 3/12/1988 Birth date: 7/24/1949
Test Date constructor with invalid values:
Month 14 invalid. Set to month 1.
Day 35 invalid. Set to day 1.
Date object constructor for date 1/1/1994
Date object destructor for date 1/1/1994
Employee object destructor: Jones, Bob
Date object destructor for date 3/12/1988
Date object destructor for date 7/24/1949
Note two additional Date objects constructed; no output since default copy constructor used.
fig07_10.cpp output (1 of 1)
Destructor for host object manager runs before destructors for member objects hireDate and birthDate.
Destructor for Employee’s member object hireDate.
Destructor for Employee‘s member object birthDate.
Destructor for Date object hire.
Destructor for Date object birth.

23. friend Functions and friend Classes
Defined outside class’s scope
Right to access non-public members
Declaring friends
Function
Precede function prototype with keyword friend
Want to make all member functions of class ClassTwo as friends of class ClassOne
Place declaration of form
friend class ClassTwo;
in ClassOne definition

24. friend Functions and friend Classes
Properties of friendship
Friendship granted, not taken
Class B friend of class A
Class A must explicitly declare class B friend
Not symmetric
Class A not necessarily friend of class B
Not transitive
Class A friend of class B
Class B friend of class C
Class A not necessarily friend of Class C

25. Precede function prototype with keyword friend.
// Fig. 7.11: fig07_11.cpp
// Friends can access private members of a class.
#include <iostream> 4
using std::cout;
using std::endl; 7
// Count class definition
class Count {
friend void setX( Count &, int ); // friend declaration 11
12 public: 13
// constructor
Count()
: x( 0 ) // initialize x to 0 {
// empty body 19
} // end Count constructor 21
fig07_11.cpp (1 of 3)
Precede function prototype with keyword friend.

26. Pass Count object since C-style, standalone function.
// output x
void print() const {
cout << x << endl; 26
} // end function print 28
29 private:
int x; // data member 31
32 }; // end class Count 33
34 // function setX can modify private data of Count
35 // because setX is declared as a friend of Count
36 void setX( Count &c, int val )
37 {
c.x = val; // legal: setX is a friend of Count 39
40 } // end function setX 41
fig07_11.cpp (2 of 3)
Pass Count object since C-style, standalone function.
Since setX friend of Count, can access and modify private data member x.

27. fig07_11.cpp (3 of 3) fig07_11.cpp output (1 of 1)
42 int main()
43 {
Count counter; // create Count object 45
cout << "counter.x after instantiation: ";
counter.print(); 48
setX( counter, 8 ); // set x with a friend 50
cout << "counter.x after call to setX friend function: ";
counter.print(); 53
return 0; 55
56 } // end main
fig07_11.cpp (3 of 3) fig07_11.cpp output (1 of 1)
Use friend function to access and modify private data member x.
counter.x after instantiation: 0
counter.x after call to setX friend function: 8

28. Using the this Pointer this pointer
Allows object to access own address
Not part of object itself
Implicit argument to non-static member function call
Implicitly reference member data and functions
Type of this pointer depends on
Type of object
Whether member function is const
In non-const member function of Employee
this has type Employee * const
Constant pointer to non-constant Employee object
In const member function of Employee
this has type const Employee * const
Constant pointer to constant Employee object

29. fig07_13.cpp (1 of 3) 1 // Fig. 7.13: fig07_13.cpp
// Using the this pointer to refer to object members.
#include <iostream> 4
using std::cout;
using std::endl; 7
class Test { 9
10 public:
Test( int = 0 ); // default constructor
void print() const; 13
14 private:
int x; 16
17 }; // end class Test 18
19 // constructor
20 Test::Test( int value )
: x( value ) // initialize x to value
22 {
// empty body 24
25 } // end Test constructor
fig07_13.cpp (1 of 3)

30. Implicitly use this pointer; only specify name of data member (x). 26
27 // print x using implicit and explicit this pointers;
28 // parentheses around *this required
29 void Test::print() const
30 {
// implicitly use this pointer to access member x
cout << " x = " << x; 33
// explicitly use this pointer to access member x
cout << "\n this->x = " << this->x; 36
// explicitly use dereferenced this pointer and
// the dot operator to access member x
cout << "\n(*this).x = " << ( *this ).x << endl; 40
41 } // end function print 42
43 int main()
44 {
Test testObject( 12 ); 46
testObject.print(); 48
return 0; 50
Implicitly use this pointer; only specify name of data member (x).
fig07_13.cpp (2 of 3)
Explicitly use this pointer with arrow operator.
Explicitly use this pointer; dereference this pointer first, then use dot operator.

31. fig07_13.cpp (3 of 3) fig07_13.cpp output (1 of 1)
51 } // end main
x = 12
this->x = 12
(*this).x = 12
fig07_13.cpp (3 of 3) fig07_13.cpp output (1 of 1)

32. Cascaded member function calls
Using the this Pointer
Cascaded member function calls
Multiple functions invoked in same statement
Function returns reference pointer to same object
{ return *this; }
Other functions operate on that pointer
Functions that do not return references must be called last

33. // Fig. 7.14: time6.h
// Cascading member function calls. 3
// Time class definition.
// Member functions defined in time6.cpp.
#ifndef TIME6_H
#define TIME6_H 8
class Time { 10
11 public:
Time( int = 0, int = 0, int = 0 ); // default constructor 13
// set functions
Time &setTime( int, int, int ); // set hour, minute, second
Time &setHour( int ); // set hour
Time &setMinute( int ); // set minute
Time &setSecond( int ); // set second 19
// get functions (normally declared const)
int getHour() const; // return hour
int getMinute() const; // return minute
int getSecond() const; // return second 24
time6.h (1 of 2)
Set functions return reference to Time object to enable cascaded member function calls.

34. time6.h (2 of 2) 25 // print functions (normally declared const)
void printUniversal() const; // print universal time
void printStandard() const; // print standard time 28
29 private:
int hour; // (24-hour clock format)
int minute; //
int second; // 33
34 }; // end class Time 35
36 #endif
time6.h (2 of 2)

35. time6.cpp (1 of 5) 1 // Fig. 7.15: time6.cpp
// Member-function definitions for Time class.
#include <iostream> 4
using std::cout; 6
#include <iomanip> 8
using std::setfill;
10 using std::setw; 11
12 #include "time6.h" // Time class definition 13
14 // constructor function to initialize private data;
15 // calls member function setTime to set variables;
16 // default values are 0 (see class definition)
17 Time::Time( int hr, int min, int sec )
18 {
setTime( hr, min, sec ); 20
21 } // end Time constructor 22
time6.cpp (1 of 5)

36. Return *this as reference to enable cascaded member function calls.
23 // set values of hour, minute, and second
24 Time &Time::setTime( int h, int m, int s )
25 {
setHour( h );
setMinute( m );
setSecond( s ); 29
return *this; // enables cascading 31
32 } // end function setTime 33
34 // set hour value
35 Time &Time::setHour( int h )
36 {
hour = ( h >= 0 && h < 24 ) ? h : 0; 38
return *this; // enables cascading 40
41 } // end function setHour 42
time6.cpp (2 of 5)
Return *this as reference to enable cascaded member function calls.
Return *this as reference to enable cascaded member function calls.

37. Return *this as reference to enable cascaded member function calls.
43 // set minute value
44 Time &Time::setMinute( int m )
45 {
minute = ( m >= 0 && m < 60 ) ? m : 0; 47
return *this; // enables cascading 49
50 } // end function setMinute 51
52 // set second value
53 Time &Time::setSecond( int s )
54 {
second = ( s >= 0 && s < 60 ) ? s : 0; 56
return *this; // enables cascading 58
59 } // end function setSecond 60
61 // get hour value
62 int Time::getHour() const
63 {
return hour; 65
66 } // end function getHour 67
Return *this as reference to enable cascaded member function calls.
time6.cpp (3 of 5)
Return *this as reference to enable cascaded member function calls.

38. time6.cpp (4 of 5) 68 // get minute value
69 int Time::getMinute() const
70 {
return minute; 72
73 } // end function getMinute 74
75 // get second value
76 int Time::getSecond() const
77 {
return second; 79
80 } // end function getSecond 81
82 // print Time in universal format
83 void Time::printUniversal() const
84 {
cout << setfill( '0' ) << setw( 2 ) << hour << ":"
<< setw( 2 ) << minute << ":"
<< setw( 2 ) << second; 88
89 } // end function printUniversal 90
time6.cpp (4 of 5)

39. time6.cpp (5 of 5) 91 // print Time in standard format
92 void Time::printStandard() const
93 {
cout << ( ( hour == 0 || hour == 12 ) ? 12 : hour % 12 )
<< ":" << setfill( '0' ) << setw( 2 ) << minute
<< ":" << setw( 2 ) << second
<< ( hour < 12 ? " AM" : " PM" ); 98
99 } // end function printStandard
time6.cpp (5 of 5)

40. // Fig. 7.16: fig07_16.cpp
// Cascading member function calls with the this pointer.
#include <iostream> 4
using std::cout;
using std::endl; 7
#include "time6.h" // Time class definition 9
10 int main()
11 {
Time t; 13
// cascaded function calls
t.setHour( 18 ).setMinute( 30 ).setSecond( 22 ); 16
// output time in universal and standard formats
cout << "Universal time: ";
t.printUniversal(); 20
cout << "\nStandard time: ";
t.printStandard(); 23
cout << "\n\nNew standard time: "; 25
fig07_16.cpp (1 of 2)
Cascade member function calls; recall dot operator associates from left to right.

41. fig07_16.cpp (2 of 2) fig07_16.cpp output (1 of 1)
// cascaded function calls
t.setTime( 20, 20, 20 ).printStandard(); 28
cout << endl; 30
return 0; 32
33 } // end main
Function call to printStandard must appear last; printStandard does not return reference to t.
fig07_16.cpp (2 of 2) fig07_16.cpp output (1 of 1)
Universal time: 18:30:22
Standard time: 6:30:22 PM
New standard time: 8:20:20 PM

42. Dynamic Memory Management with Operators new and delete
Control allocation and deallocation of memory
Operators new and delete
Include standard header <new>
new
Time *timePtr;
timePtr = new Time;
Creates object of proper size for type Time
Error if no space in memory for object
Calls default constructor for object
Returns pointer of specified type
Providing initializers
double *ptr = new double( );
Time *timePtr = new Time( 12, 0, 0 );
Allocating arrays
int *gradesArray = new int[ 10 ];

43. Dynamic Memory Management with Operators new and delete
Destroy dynamically allocated object and free space
Consider
delete timePtr;
Operator delete
Calls destructor for object
Deallocates memory associated with object
Memory can be reused to allocate other objects
Deallocating arrays
delete [] gradesArray;
Deallocates array to which gradesArray points
If pointer to array of objects
First calls destructor for each object in array
Then deallocates memory

44. static Class Members static class variable “Class-wide” data
Property of class, not specific object of class
Efficient when single copy of data is enough
Only the static variable has to be updated
May seem like global variables, but have class scope
Only accessible to objects of same class
Initialized exactly once at file scope
Exist even if no objects of class exist
Can be public, private or protected

45. Accessing static class variables
static Class Members
Accessing static class variables
Accessible through any object of class
public static variables
Can also be accessed using binary scope resolution operator(::)
Employee::count
private static variables
When no class member objects exist
Can only be accessed via public static member function
To call public static member function combine class name, binary scope resolution operator (::) and function name
Employee::getCount()

46. static member functions
static Class Members
static member functions
Cannot access non-static data or functions
No this pointer for static functions
static data members and static member functions exist independent of objects

47. static data member is class-wide data.
// Fig. 7.17: employee2.h
// Employee class definition.
#ifndef EMPLOYEE2_H
#define EMPLOYEE2_H 5
class Employee { 7
public:
Employee( const char *, const char * ); // constructor
~Employee(); // destructor
const char *getFirstName() const; // return first name
const char *getLastName() const; // return last name 13
// static member function
static int getCount(); // return # objects instantiated 16
17 private:
char *firstName;
char *lastName; 20
// static data member
static int count; // number of objects instantiated 23
24 }; // end class Employee 25
employee2.h (1 of 2)
static member function can only access static data members and member functions.
static data member is class-wide data.

48. employee2.h (2 of 2) employee2.cpp (1 of 3)
26 #endif
// Fig. 7.18: employee2.cpp
// Member-function definitions for class Employee.
#include <iostream> 4
using std::cout;
using std::endl; 7
#include <new> // C++ standard new operator
#include <cstring> // strcpy and strlen prototypes 10
11 #include "employee2.h" // Employee class definition 12
13 // define and initialize static data member
14 int Employee::count = 0; 15
16 // define static member function that returns number of
17 // Employee objects instantiated
18 int Employee::getCount()
19 {
return count; 21
22 } // end static function getCount
employee2.h (2 of 2) employee2.cpp (1 of 3)
Initialize static data member exactly once at file scope.
static member function accesses static data member count.

49. new operator dynamically allocates space.23
24 // constructor dynamically allocates space for
25 // first and last name and uses strcpy to copy
26 // first and last names into the object
27 Employee::Employee( const char *first, const char *last )
28 {
firstName = new char[ strlen( first ) + 1 ];
strcpy( firstName, first ); 31
lastName = new char[ strlen( last ) + 1 ];
strcpy( lastName, last ); 34
count; // increment static count of employees 36
cout << "Employee constructor for " << firstName
<< ' ' << lastName << " called." << endl; 39
40 } // end Employee constructor 41
42 // destructor deallocates dynamically allocated memory
43 Employee::~Employee()
44 {
cout << "~Employee() called for " << firstName
<< ' ' << lastName << endl; 47
employee2.cpp (2 of 3)
new operator dynamically allocates space.
Use static data member to store total count of employees.

50. Operator delete deallocates memory. employee2.cpp (3 of 3)
delete [] firstName; // recapture memory
delete [] lastName; // recapture memory 50
count; // decrement static count of employees 52
53 } // end destructor ~Employee 54
55 // return first name of employee
56 const char *Employee::getFirstName() const
57 {
// const before return type prevents client from modifying
// private data; client should copy returned string before
// destructor deletes storage to prevent undefined pointer
return firstName; 62
63 } // end function getFirstName 64
65 // return last name of employee
66 const char *Employee::getLastName() const
67 {
// const before return type prevents client from modifying
// private data; client should copy returned string before
// destructor deletes storage to prevent undefined pointer
return lastName; 72
73 } // end function getLastName
Operator delete deallocates memory.
employee2.cpp (3 of 3)
Use static data member to store total count of employees.

51. new operator dynamically allocates space.
// Fig. 7.19: fig07_19.cpp
// Driver to test class Employee.
#include <iostream> 4
using std::cout;
using std::endl; 7
#include <new> // C++ standard new operator 9
10 #include "employee2.h" // Employee class definition 11
12 int main()
13 {
cout << "Number of employees before instantiation is "
<< Employee::getCount() << endl; // use class name 16
Employee *e1Ptr = new Employee( "Susan", "Baker" );
Employee *e2Ptr = new Employee( "Robert", "Jones" ); 19
cout << "Number of employees after instantiation is "
<< e1Ptr->getCount(); 22
fig07_19.cpp (1 of 2)
new operator dynamically allocates space.
static member function can be invoked on any object of class.

52. Operator delete deallocates memory.
cout << "\n\nEmployee 1: "
<< e1Ptr->getFirstName()
<< " " << e1Ptr->getLastName()
<< "\nEmployee 2: "
<< e2Ptr->getFirstName()
<< " " << e2Ptr->getLastName() << "\n\n"; 29
delete e1Ptr; // recapture memory
e1Ptr = 0; // disconnect pointer from free-store space
delete e2Ptr; // recapture memory
e2Ptr = 0; // disconnect pointer from free-store space 34
cout << "Number of employees after deletion is "
<< Employee::getCount() << endl; 37
return 0; 39
40 } // end main
fig07_19.cpp (2 of 2)
Operator delete deallocates memory.
static member function invoked using binary scope resolution operator (no existing class objects).

53. Number of employees before instantiation is 0
Employee constructor for Susan Baker called.
Employee constructor for Robert Jones called.
Number of employees after instantiation is 2
Employee 1: Susan Baker
Employee 2: Robert Jones
~Employee() called for Susan Baker
~Employee() called for Robert Jones
Number of employees after deletion is 0
fig07_19.cpp output (1 of 1)

54. Data Abstraction and Information Hiding
Classes hide implementation details from clients
Example: stack data structure
Data elements added (pushed) onto top
Data elements removed (popped) from top
Last-in, first-out (LIFO) data structure
Client only wants LIFO data structure
Does not care how stack implemented
Data abstraction
Describe functionality of class independent of implementation

55. Data Abstraction and Information Hiding
Abstract data types (ADTs)
Approximations/models of real-world concepts and behaviors
int, float are models for a numbers
Data representation
Operations allowed on those data
C++ extensible
Standard data types cannot be changed, but new data types can be created

56. Forward class declaration
Proxy Classes
Proxy class
Hide implementation details of another class
Knows only public interface of class being hidden
Enables clients to use class’s services without giving access to class’s implementation
Forward class declaration
Used when class definition only uses pointer to another class
Prevents need for including header file
Declares class before referencing
Format:
class ClassToLoad;

57. public member function.
// Fig. 7.20: implementation.h
// Header file for class Implementation 3
class Implementation { 5
public: 7
// constructor
Implementation( int v )
: value( v ) // initialize value with v {
// empty body 13
} // end Implementation constructor 15
// set value to v
void setValue( int v ) {
value = v; // should validate v 20
} // end function setValue 22
implementation.h (1 of 2)
public member function.

58. public member function. implementation.h (2 of 2)
// return value
int getValue() const {
return value; 27
} // end function getValue 29
30 private:
int value; 32
33 }; // end class Implementation
public member function.
implementation.h (2 of 2)

59. Pointer to Implementation object requires forward class declaration.
// Fig. 7.21: interface.h
// Header file for interface.cpp 3
class Implementation; // forward class declaration 5
class Interface { 7
public:
Interface( int );
void setValue( int ); // same public interface as
int getValue() const; // class Implementation
~Interface(); 13
14 private: 15
// requires previous forward declaration (line 4)
Implementation *ptr; 18
19 }; // end class Interface
interface.h (1 of 1)
Pointer to Implementation object requires forward class declaration.
Provide same public interface as class Implementation; recall setValue and getValue only public member functions.

60. Maintain pointer to underlying Implementation object.
// Fig. 7.22: interface.cpp
// Definition of class Interface
#include "interface.h" // Interface class definition
#include "implementation.h" // Implementation class definition 5
// constructor
Interface::Interface( int v )
: ptr ( new Implementation( v ) ) // initialize ptr {
// empty body 11
12 } // end Interface constructor 13
14 // call Implementation's setValue function
15 void Interface::setValue( int v )
16 {
ptr->setValue( v ); 18
19 } // end function setValue 20
Maintain pointer to underlying Implementation object.
interface.cpp (1 of 2)
Proxy class Interface includes header file for class Implementation.
Invoke corresponding function on underlying Implementation object.

61. Invoke corresponding function on underlying Implementation object.
21 // call Implementation's getValue function
22 int Interface::getValue() const
23 {
return ptr->getValue(); 25
26 } // end function getValue 27
28 // destructor
29 Interface::~Interface()
30 {
delete ptr; 32
33 } // end destructor ~Interface
Invoke corresponding function on underlying Implementation object.
interface.cpp (2 of 2)
Deallocate underlying Implementation object.

62. fig07_23.cpp (1 of 1) fig07_23.cpp output (1 of 1)
// Fig. 7.23: fig07_23.cpp
// Hiding a class’s private data with a proxy class.
#include <iostream> 4
using std::cout;
using std::endl; 7
#include "interface.h" // Interface class definition 9
10 int main()
11 {
Interface i( 5 ); 13
cout << "Interface contains: " << i.getValue()
<< " before setValue" << endl; 16
i.setValue( 10 ); 18
cout << "Interface contains: " << i.getValue()
<< " after setValue" << endl; 21
return 0; 23
24 } // end main
fig07_23.cpp (1 of 1) fig07_23.cpp output (1 of 1)
Only include proxy class header file.
Create object of proxy class Interface; note no mention of Implementation class.
Invoke member functions via proxy class object.
Interface contains: 5 before setValue
Interface contains: 10 after setValue